The way I would set up the comp is to start the first 15 frame transition on frame 1, finish it on frame 16, then start the next transition on frame 18 and finish it on frame 33, then start the next transition of frame 35 and finish it on frame 50, then start the last transition on frame 52 and finish it by ending on the first layer at frame 67. The total length of the banner is 45 seconds plus 4 * 15 frame transitions. Fifteen seconds layer there is a 10 frame transition to the next frame and it holds for 15 seconds, then there is another 10 frame transition to the next frame, then that frame (image) holds for another 15 seconds, then the third transition for 10 frames to the final frame before the last transition back to the first frame.

Let's say you are designing an animated banner for a website. There is never any need for duplicate frames in an animated gif. You can have that frame last for 30 seconds if you like. Inside Photoshop you can set any frame for an animated gif to any duration. Let me talk about the no duplicate frames.

Use the Photoshop File menu to export an animated gif for the web using the Legacy option.

In Photoshop, adjust the duration of any frames that I want to last longer than one frame (read the explanation).Render the file and open it in Photoshop.Add the comp to the Render Queue and choose the Lossless With Alpha preset in the Output Module settings.Make sure that are no duplicate frames in the animation (see details below).Set the comp frame size to an even number of pixels high and wide because Gif's work better that way.Set the comp frame rate to 8, 12, or 15 - there is no need for any higher frame rate in an animated gif.

If I need an animated gif from an After Effects comp I do the following:

Lossless cut review